1. Field of the Invention
This invention generally relates to a method and apparatus for mounting semiconductor chips on a printed circuit board and, more particularly, to a method and apparatus for directly attaching a semiconductor chip to a circuit pattern on a printed circuit board.
2. Description of the Prior Art
Semiconductor chips comprise solid state circuitry formed in a semiconductive material such as germanium or silicon. These chips are often referred to as integrated circuit (IC) chips. Integrated circuit chips are widely used in a variety of electronic equipment, from computers and communication equipment to consumer electronics and power electronics. Integrated circuit chips typically have a multiplicity of contact terminations on a side that must be electrically interconnected to external circuitry on a printed circuit board.
The conventional method of mounting integrated circuit chips to printed circuit boards makes use of packages such as the dual in-line package (DIP), the single in-line package (SIP), or the leadless chip carrier package. In the typicl DIP or SIP, the contact terminations on the integrated circuit chip are connected to a lead frame comprised of small conductive leads. Some lead frames are defined by lead patterns etched onto a plastic tape. The conductive leads are connected to larger terminal leads which are integral with the package. The terminal leads are then plugged into circuit terminals on the circuit board and soldered. In a leadless chip carrier package, the leads connected to terminals on the integrated circuit chip are conected to terminals on the package which are connected directly with the circuit pattern on the printed circuit board by soldering.
Tape automated bonding (TAB) is a packaging process in which conductive leads etched on a polymer film are automatically bonded to the contact terminals on the integrated circuit chip. A TAB machine is supplied with integrated circuit chips, a long lead frame tape which sometimes comprises a 35 mm film strip with conductive leads extending over rectangular openings in each frame of the film, and package casings with integral terminals. The TAB machine will automatically produce DIPs or SIPs ready for mounting to a circuit board.
The size of the DIP, SIP, or leadless chip carrier package is many times the size of an integrated circuit chip. The sizes of these packages were dictated in part by the technology for assembly which existed at the time the packages were developed. These packages tend to take up too much space on a printed circuit board. Therefore, efforts have been made to increase the packing density of integrated circuit chips on printed circuit boards such that a single board could perform more functions. Flip chip technology eliminates packages by simply bonding the integrated circuit chips directly to the printed circuit board. Solder balls are placed on the chip terminals and then chips are flipped over for connecting the solder balls to terminals on the printed circuit board. Eliminating the large package allows more chips to be mounted on a printed circuit board. Closer packing of integrated circuit chips reduces the signal delay between each chip.
However, flip chip technology requires precise placement of chips on the board and equipment that will delicately handle the integrated circuit chip when applying the solder ball and flipping the chip. It is also difficult to bond the flip chip to the terminals on the printed circuit board if the terminals are not the same height. Moreover, a tremendous amount of heat is developed by closely spaced integrated circuit chips during operation of the printed circuit board. Excess heat will interfere with operation of integrated circuit chips; therefore, a need exists for cooling each chip and isolating each chip from the heat effects of neighboring chips.
The most serious drawback of flip chip technology is the inability to permit easy chip replacement. Because the integrated circuit chips are soldered closely to the board it is difficult to remove a defective chip and, therefore, it is often necessary to dispose of the entire printed circuit board if circuit testing indicates chip defects or chip connection defects.